In the catecholamine biosynthetic pathway, tyrosine is converted in three steps to norephinephrine (NE). Intermediates are dihydroxyphenylalanine (DOPA) and dopamine (DA). The latter is hydroxylated to norepinephrine by dopamine-.beta.-hydroxylase (DBH) in the presence of oxygen and ascorbic acid.
Inhibition of catecholamine activity has been found to decrease hypertension. See, for example, Matta et al., Clin. Pharm. Ther. 14, 541 (1973), and Teresawa et al., Japan Circ. J. 35, 339 (1971). Weinshilboum, Mayo Clin. Proc. 55, 39 (1980), reviews compounds which inhibit catecholamine activity by interfering with adrenergic receptors. Alternatively, the catecholamine biosynthetic pathway can be suppressed at any of the three steps, resulting in decreased levels of NE. In addition to decreasing hypertension, inhibitors of NE synthesis are active as diuretics, natriuretics, cardiotonics and vasodilators. Inhibition of DBH activity can have the added advantage of increasing levels of DA, which as reported by Ehrreich et al., "New Antihypertensive Drugs," Spectrum Publishing, 1976, pp. 409-432, has been found to have selective vasodilator activity at certain concentrations.
DBH inhibitors have also been shown to reduce or prevent formation of gastric ulcers in rats by Hidaka et al., "Catecholamine and Stress," edit. by Usdin et al, Permagon Press, Oxford, 1976, pp. 159-165 and by Osumi et al., Japan. J. Pharmacol. 23, 904 (1973).
A number of DBH inhibitors are known. These are generally divided into two classes, namely, metal chelating agents, which bind to copper in the enzyme, and phenethylamine analogues. Rosenberg et al., "Essays in Neurochemistry and Neuropharmacology, Vol. 4," edit. by Youdim et al., John Wiley & Sons, 1980, pp. 179-192, and Goldstein, Pharmacol. Rev. 18(1), 77 (1966), review DBH inhibitors. The former report that many of the potent DBH inhibitors have a hydrophobic side chain of size comparable to the aromatic ring of DA, leading the authors to suggest that incorporation of a terminal hydroxyl group on a 4- to 6-carbon side chain on a phenethylamine analogue might yield a potent inhibitor.
Known inhibitors include:
5-alkylpicolinic acids [See, Suda et al., Chem. Pharm. Bull. 17, 2377 (1969); Umezawa et al., Biochem. Pharmacol. 19, 35 (1969); Hidaka et al., Mol. Pharmacol. 9, 172 (1973); Miyano et al., Chem. Pharm. Bull. 26, 2328 (1978); Miyano et al., Heterocycles 14, 755 (1980); Claxton et al., Eur. J. Pharmacol. 37, 179 (1976)];
BRL 8242 [See, Claxton et al., Eur. J. Pharmacol. 37, 179 (1976)];
1-alkyl-2-mercaptoimidazole [See, Hanlon et al., Life Sci. 12, 417 (1973); Fuller et al., Adv. Enzyme Regul. 15, 267 (1976)];
substituted thioureas [See, Johnson et al., J. Pharmacol. Exp. Ther. 168, 229 (1969)]; and
benzyloxyamine and benzylhydrazine [See, Creveling et al., Biochim. Biophys. Acta 64, 125 (1962); Creveling et al., Biochim. Biophys. Res. Commun: 8, 215 (1962); van der Schoot et al., J. Pharmacol. Exp. Ther. 141, 74 (1963); Bloom, Ann. N.Y. Acad. Sci. 107, 878 (1963)].
All of the above compounds except benzyloxyamine and benzylhydrazine apparently owe their inhibitory effect to metal chelating properties. Alkyl derivatives of 2-mercaptoimidazole are more potent, presumably due to non-specific interaction of the alkyl substituent with the enzyme. Benzyloxyamine and benzylhydrazine are phenethylamine derivatives which apparently act as competitive inhibitors.
In addition to the above compounds, Runti et al., Il Farmaco Ed. Sc. 36, 260 (1980), report that other fusaric acid derivatives and analogues can inhibit DBH. These include phenopicolinic acid, which is reported to have twice the inhibitory activity of fusaric acid, and 5-(4-chlorobutyl)picolinic acid, and others such as substituted amides of fusaric acid and acids and amides of 5-butyroylpicolinic acid, 5-aminopicolinic acid and 5-hydrazinopicolinic acid, and derivatives thereof.
Hidaka et al., Molecular Pharmacology, 9, 172-177 (1972) report that 5-(3,4-dibromo)butyl picolinic acid and 5-(dimethyldithiocarbamoyl)methyl picolinic acid are DBH inhibitors.
Bupicomide, 5-(n-butyl)picolinamide, is reported by Enrreich et al., "New Antihypertensive Drugs," Spectrum Publications, 1976, pg. 409-432, to be a DBH inhibitor and to have antihypertensive activity.
Friedman et al. Psychosomatic Med. 40, 107 (1978), report that patients treated with alpha-methyl-DOPA, guanethidine and reserpine, but not propanolanol and diuretics, have lowered DBH levels, although significance of the observation is uncertain.
DBH hydroxylates a variety of phenethylamine substrates. Rosenberg et al., "Essays in Neurochemistry and Neuropharmacology, Vol. 4," edit. by Youdim et al. John Wiley & Sons, 1980, pp. 163-209, extensively review the chemistry of DBH, including, at pp. 176-179 and 196-202, proposed mechanisms of action. There is not yet a known, satisfactory model of the mechanism of action of DBH.
Although there are many known inhibitors of DBH, none of these agents has found clinical application because of non-specific, often toxic, properties they possess. Fusaric acid, for example, has been found to be hepatotoxic. See, for example, Teresawa et al., Japan. Cir. J. 35, 339 (1971) and references cited therein. Presumably, the picolinic acid structure interacts with a number of metalloproteins and enzymes in non-specific fashion to produce observed side effects.
In U.K. specification No. 1,155,580 are disclosed compounds having the formula: ##STR2## wherein R.sup.2 and R.sup.3 can be H and R.sup.1 can be substituted phenyl. The compounds are said to have analgesic, anti-inflammatory and antipyretic properties. Gebert et al., U.S. Pat. No. 3,915,980, disclose such compounds wherein R.sup.1 can be phenyl or phen (C.sub.1-3) alkyl, as intermediates to imidazolyl-2-thioalkanoic acid esters.
Iverson, Acta Chem. Scand. 21, 279 (1967) reports a compound having the formula: ##STR3## wherein R can be --CO.sub.2 H or --CH.sub.2 NHC.sub.6 H.sub.5, but does not report a pharmaceutical use for the compound.